// Add type declaration for the custom Math function
declare global {
  interface Math {
    easeInOutQuad: (t: number, b: number, c: number, d: number) => number
  }
}

Math.easeInOutQuad = function(t: number, b: number, c: number, d: number): number {
  t /= d / 2
  if (t < 1) {
    return c / 2 * t * t + b
  }
  t--
  return -c / 2 * (t * (t - 2) - 1) + b
}

// requestAnimationFrame for Smart Animating http://goo.gl/sx5sts
const requestAnimFrame = (function(): (callback: FrameRequestCallback) => number {
  return window.requestAnimationFrame ||
        //  window.webkitRequestAnimationFrame ||
        //  window.mozRequestAnimationFrame ||
         function(callback: FrameRequestCallback): number {
           return window.setTimeout(callback, 1000 / 60)
         }
})()

/**
 * Because it's so fucking difficult to detect the scrolling element, just move them all
 * @param {number} amount
 */
function move(amount: number): void {
  document.documentElement.scrollTop = amount
  document.documentElement.scrollTop = amount
  document.body.scrollTop = amount
}

function position(): number {
    return (
    window.pageYOffset ||
    document.documentElement.scrollTop ||
    document.body.scrollTop ||
    0
  )
}

/**
 * @param {number} to
 * @param {number} duration
 * @param {Function} callback
 */
export function scrollTo(to: number, duration?: number, callback?: () => void): void {
  const start = position()
  const change = to - start
  const increment = 20
  let currentTime = 0
  duration = (typeof (duration) === 'undefined') ? 500 : duration
  const animateScroll = function() {
    // increment the time
    currentTime += increment
    // find the value with the quadratic in-out easing function
    const val = Math.easeInOutQuad(currentTime, start, change, duration)
    // move the document.body
    move(val)
    // do the animation unless its over
    if (currentTime < duration) {
      requestAnimFrame(animateScroll)
    } else {
      if (callback && typeof (callback) === 'function') {
        // the animation is done so lets callback
        callback()
      }
    }
  }
  animateScroll()
}
